1. Field
The present invention relates to an electrowetting display device and a method for manufacturing the same, and more particularly to an electrowetting display device having banks of different hydrophilicities/hydrophobicities and a method for manufacturing the same.
2. Discussion of the Background
An electrowetting effect refers to the change of contact angle between a conductive hydrophobic fluid and a hydrophobic layer positioned beneath the fluid, when a voltage is selectively applied to the fluid. Specifically, the smaller the voltage applied to the hydrophobic fluid is, the smaller the contact angle between the hydrophobic fluid and the hydrophobic layer becomes. The smaller the contact angle is, in turn, the larger the surface area of the hydrophobic fluid on the hydrophobic layer becomes.
An electrowetting display device is based on such an electrowetting effect. FIGS. 1A and 1B are sectional views of a pixel of a conventional electrowetting display device.
The electrowetting display device 10 includes a first substrate 100, a hydrophobic fluid 210 positioned on the first substrate 100, a hydrophilic fluid 230 positioned on the hydrophobic fluid 210 and which is immiscible with the hydrophobic fluid 210, and a second substrate 190 positioned over the hydrophilic fluid 230.
The first substrate 100 includes a first basic substrate 110, a first pixel electrode 120 formed on the first basic substrate 110, and a hydrophobic layer 130 formed on the pixel electrode 120. Banks 140 are formed on parts of the hydrophobic layer 130 to limit the movement of the hydrophobic fluid 210. The hydrophobic fluid 210 includes a pigment of a predetermined color. When the pixel electrode 120 is provided with no voltage, the hydrophobic fluid 210 is distributed over the entire hydrophobic layer 130, as shown in FIG. 1A, and the pixel displays the color of the pigment. The hydrophilic fluid 230 is transparent and is arranged on the hydrophobic fluid 210.
In order for the display device 10 to display a predetermined image, voltages of a predetermined gradation level are applied to the pixel electrode 120 of each pixel. When a voltage is applied to the pixel electrode 120, as shown in FIG. 1B, a part of the hydrophobic fluid 210 at the center of the layer is broken up, and then the hydrophobic fluid 210 moves towards the banks 140 positioned on edges of the pixel electrode. In order to guarantee that the breakup occurs at a constant position, the pixel further includes an auxiliary electrode 125 positioned separate from the pixel electrode 120 and provided with a voltage different from the pixel electrode 120. However, even the existence of the auxiliary electrode 125 cannot guarantee that the hydrophobic fluid 210 consistently brakes up.
Such breakup of the hydrophobic fluid 210 requires that a high voltage (e.g. 30 volts) be applied to the pixel electrode. However, high-voltage driving of the display device increases its power consumption, and the resulting electrolysis of fluids creates bubbles of oxygen or hydrogen gas as impurities inside the display device, degrading the display quality.
A satisfactory level of image display quality may require that the hydrophobic fluid of each pixel be distributed in the same position and area with regard to a predetermined gradation level. However, the inconsistency of the initial breakup position of the hydrophobic fluid makes it difficult to adjust the position and area of the hydrophobic fluid. Furthermore, the position and area of the hydrophobic fluid change, even if the pixel voltage is the same, during splitting and reunion of the hydrophobic fluid (hysteresis phenomenon), making it difficult to determine a voltage level with regard to a predetermined gradation level. Therefore, there is a need to design a display device in such a manner that fluids can also be driven at a low voltage, no unwanted gas is produced, and the fluids have the same position and area at a predetermined gradation level, as well as a method for manufacturing the same.